A phylum of about 180 species of marine coelomate invertebrates, the arrow worms, in which the head bears hooks for catching prey and the trunk and tail support paired lateral and tail fins. They lack organs for excretion, circulation, and respiration and are hermaphrodite. Fossil evidence shows that arrow worms were abundant 500 million years ago but their relationship to other animal groups has long been uncertain. They are now regarded as protostomes, albeit ones with developmental traits ancestral to all triploblastic animals.